1. Field of the Invention
This invention relates to an automatic exposure control device adapted for a video camera or the like.
2. Description of the Related Art
Heretofore, a video camera or the like has been using an automatic exposure control device which controls the iris of an optical system and an automatic gain control (hereinafter referred to as AGC) circuit to keep the level of a video signal constant.
FIGS. 1, 2 and 3 of the accompanying drawings show the arrangement of the automatic exposure control device of the conventional video camera. FIG. 1 is a block diagram mainly showing in outline the typical arrangement of the exposure control system of the video camera. The illustration includes a photo-taking lens 1; an iris 2 which is arranged to adjust the quantity of incident light coming through the photo-taking lens 1; a CCD 3 serving as an image sensor which is arranged to photo-electric convert into a video signal the image of an object formed through the lens 1 on the image sensing plane thereof and to output the video signal; a buffer amplifier 4 which is disposed on the output side of the CCD 3; an AGC circuit 5 which performs gain control to keep constant the level of the video signal supplied from the buffer amplifier 4; and a video signal processing circuit 6 which is arranged to perform a processing operation on the level-adjusted video signal supplied from the AGC circuit 5, including gamma correction, a blanking process, addition of a synchronizing signal, etc. Further, a light measuring circuit 7a is arranged to make the incident light quantity at an apposite value by detecting the level of the signal output from the buffer amplifier 4 and by controlling the iris 2 via an iris driving circuit 8 according to the detected level. In other words, the light measuring circuit 7a is arranged to control the iris 2 in such a way as to keep the output signal level of the buffer amplifier 4 constant. Another light measuring circuit 7b is arranged to perform gain control to keep the output signal level of the AGC circuit 5 constant by detecting the output signal level of the AGC circuit 5 and by feeding back an error signal according to the result of the level detection. Generally, averaging circuits are employed as these light measuring circuits 7a and 7b. The details of each of the averaging circuits are as shown by way of example in FIG. 2. The internal circuit arrangements of the two light measuring circuits 7a and 7b are the same. Therefore, only the light measuring circuit 7a is described below:
Referring to FIG. 2, the light measuring circuit 7a includes a buffer amplifier 71a, a resistor R1 and a capacitor C1. The resistor R1 and the capacitor C1 constitute a time constant circuit 72a. The video output signal 4' which has been output from the buffer amplifier 4 is averaged at a time constant .tau. by the time constant circuit 72a. The averaged signal is supplied to the iris driving circuit 8 through the buffer amplifier 71a. The iris driving circuit 8 then controls the iris 2 in accordance with the averaged signal. Meanwhile, the other light measuring circuit 7b is likewise supplied with a video output signal 5' output from the AGC circuit 5, which is averaged by the light measuring circuit 7b at the time constant .tau.. The averaged signal is supplied to the AGC circuit 5.
In the event of a high contrast image, however, the light measurement performed by the above-stated light measuring circuit results in an entirely blackened picture under a back light condition or in an entirely whitened picture under a forward light condition. In both cases, the light measurement gives a very unnatural picture.
To solve this problem, a so-called in-frame light measuring method has been proposed. In accordance with this method, a light measuring area is set at a part of the image sensing plane and the light measurement is carried out with the light measuring area weighted, so that an exposure can be adjusted to a main object to be photographed. FIG. 3 shows by way of example the arrangement of such a light measuring circuit. The light measuring circuit of FIG. 3 differs from that of FIG. 2 in that an analog switch 91 is arranged before a time constant circuit within the light measuring circuit 9 to provide a gate for setting a part of the image sensing plane as a light measuring area. The light measuring circuit 9 includes a buffer amplifier 92, a resistor R1 and a capacitor Cl which constitute a time constant circuit in the same manner as in the case of FIG. 2. In each of the video signals 4' and 5', a signal part corresponding to the light measuring area is gated by means of this analog switch 91. The video signal obtained during the passing period of this gate is averaged by the time constant circuit 93. A control signal 94 which is generated in synchronism with an H and V composite signal consisting of horizontal and vertical synchronizing signals is arranged to control the ON/OFF switching action of this analog switch 91. The analog switch 91 is thus arranged to be turned on for a video signal part corresponding to the light measuring area of the image sensing plane to have this signal part alone allowed to pass there.
In a case where the video camera has a light measuring circuit arranged in accordance with the so-called in-frame light measuring method as shown in FIG. 3, a time constant of the time constant circuit is generally arranged to be several tens of msecs which corresponds to several frames of a TV signal. This arrangement enables the signal level of an image plane to be kept almost constant to give a natural picture in terms of visual sensation even in the event of a sudden change in light quantity resulting from a quick panning or tilting movement of the camera. However, in cases where a photographing operation is to be performed with the camera fixed to keep a picture frame stationary, the signal level within a light measuring area would change to result in a sudden level change of the video signal if, for example, the object being photographed happens to laterally move relative to the light measuring area. In such a case, the image becomes very unnatural. This phenomenon saliently appears particularly in the event of a great contrast between the object to be photographed and the background thereof. Further, an unnatural picture likewise appears also in a case where the camera is slowly moved by a panning or tilting action to have the light measuring area moved across an object of a high relative contrast. This problem may be miti-gated by setting the above-stated time constant at such a value that allows the video signal to slowly change in terms of visual sensation as mentioned in the foregoing. However, that arrangement degrades the follow-up capability of the video signal level in the event of a quick panning or tilting movement of the video camera.
U.S. patent application Ser. No. 264,204 filed Oct. 28, 1988 and assigned to the assignee of the present invention has disclosed an exposure control device of the above-stated kind. The exposure control device is arranged to change the position of the light measuring area following the movement of the image of an object to be photographed and also to change the size of the light measuring area according to the size of the object's image. This arrangement enables the device to adequately accomplish exposure control in accordance with the state of the object.
However, the above-stated patent application has disclosed nothing concerning the speed of response to a change occurring in the exposure conditions as a result of a change in the object.